Redox cycling is an electrochemical process in which electrochemically active molecules are repeatedly oxidized and reduced. These reactions take place between two electrodes located in close proximity to one another. For this purpose, appropriate oxidizing and reducing potentials are applied to the electrodes, whereby the molecules are directly oxidized or reduced upon contact with the electrodes. When the molecule thereafter diffuses to the other electrode, the respective reverse process (reduction/oxidation) takes place. This iterative process causes a charge transport between the electrodes through each individual molecule, which results in amplification of the measurable overall signal.
In the prior art, these types of sensors are produced by way of optical lithography or electron beam lithography. Several designs for redox cycling sensors and methods for the production thereof have been published.
A sensor comprising laterally juxtaposed electrodes is known from Goluch et al. (2009). These so-called “interdigitated electrodes” have a distance in the range of nanometers to micrometers between the individual digits. The method used for production is electron beam lithography, which provides for complex lift-off and etching processes.
Electrodes disposed in the Z axis, which is to say on top of one another, are known from Wolfrum et al. (2008), Kätelhön et al. (2010) and Zevenbergen et al. (2011), and have a nanoscale gap serving as what is known as a “nanocavity” or “nanochannel” therebetween. These are produced by way of electron beam lithography or optical lithography, which comprises multiple etching steps and provides for the removal of a sacrificial chromium layer, among other things.
Hüske et al. (2014) disclose electrodes disposed in the Z axis, which is to say on top of one another, which comprise a nanoscale dielectric between the two electrodes. The electrodes are produced by way of optical lithography and electron beam lithography. The production method includes multiple deposition and etching steps, but also a so-called “self-assembly” step as a result of the anodization of aluminum.
Electrodes disposed in the Z axis, which is to say on top of one another, which include a microscale gap, are known from Gross et al. (2015). The electrodes are produced by adhesively bonding two separate electrodes, comprising a thick layer of dielectric therebetween.
Disadvantages in the Prior Art Include:
a. Complex manufacturing methods, using photolithography or electron beam lithography, which does not allow economically useful scaling of production.
b. High costs during production, or non-scalable production processes.
c. All previous designs require the electrodes and/or an intermediate layer to be post-modified or provided with recognition molecules, such as antibodies, aptamers and the like, so as to detect biomolecules, such as antigens, antibodies, DNA, or the like. These are time-consuming and cost-intensive.
d. Inadequate sensitivity of microscale redox cycling sensors.
e. Mechanical instability if gaps are very small.
f. Etching steps or multiple chemical steps during fabrication.
g. Some steps of existing methods are difficult to reproduce.
Additional Disadvantages of the Methods Include:                1. Regarding Goluch et al. 2009: The method results in very high production costs due to electron beam lithography being employed. Additionally, delamination of the individual digits occurs with very small lateral distances. If distances are larger, the sensitivity is insufficient due to lower efficiency of the redox cycling process.        2. Regarding Wolfrum et al., Kätelhön et al. 2010, and Zevenbergen et al.: These methods likewise cause high production costs due to the optical or electron beam lithography employed. Moreover, multiple lithography processes having very good alignment are required for the sensors to function. Very small gaps and/or laterally wide gaps result in mechanical instability. In addition, etching steps are necessary for removal of the sacrificial layer, which allow the formation of the intermediate layer after etching.        3. Regarding Hüske et al. 2014: This is a production method that is difficult to reproduce due to the variations in the anodization steps. The method alternatively uses electron beam lithography, which results in very high production costs. Multiple chemical steps and washing steps are needed during fabrication.        4. Regarding Gross et al. 2015: This is a production method that is difficult to reproduce, having very low efficiency in the redox cycling process.        